Conventionally, in display devices for, for example, displaying numerical figures or letters or performing planar light emission using point light sources such as light emitting diodes or linear light sources such as fluorescent lamps or cathode ray tubes, optical diffusion films are used which diffuse the transmitted light from the point or linear light sources to make the light appear to be from planar light sources.
In the field of flat panel displays (FPDs), it is desired to alleviate the nonuniformity in the luminance and to improve the luminance in order to realize higher display performances.
A general backlight unit of a liquid crystal display includes a waveguide plate having optical diffusion dots printed on a rear surface thereof, a light source (cathode ray tube, etc.) located on one side or both of two sides of the waveguide plate, an optical diffusion sheet provided on the waveguide plate, a lens film (prism sheet) provided optionally on an upper surface, or both on upper and lower surfaces, of the optical diffusion sheet, and the like.
The optical diffusion sheet incorporated into such a backlight unit uniformly diffuses light from the waveguide plate to prevent the dots from being visible on the display screen and uniformly releases the diffused light toward the liquid crystal panel while suppressing the optical loss.
Several types of such an optical diffusion sheet have been conventionally proposed. In order to achieve the above-described object, for example, Japanese Laid-Open Patent Publication No. 11-337711 proposes a material using a highly optical diffusion agent which is formed of at least one material selected from organic materials such as PMMA (polymethyl methacrylate), polyacrylonitrile, polyester, silicone, polyethylene, epoxy, melamine-formaldehyde condensate, benzoguanamine-formaldehyde condensate, benzoguanamine-melamine-formaldehyde condensate and the like and of spherical or true spherical particles having a particle diameter of 1 to 20 μm.
Japanese Laid-Open Patent Publication No. 2003-107214 proposes an optical diffusion sheet including optical diffusion layers stacked on a transparent support. The optical diffusion layers contain a binder resin and resin particles and have a concaved and convexed surface. The above-identified publication reports that the total light transmittance of the sheet is 70.0% or higher, the haze is 80.0% or higher, the clarity of the image obtained by transmission is 21.0% or higher and lower than 25.0%.
However, the optical diffusion sheets as disclosed in Japanese Laid-Open Patent Publications Nos. 11-337711 and 2003-107214 have the problems that, for example, the clarity is insufficient, the display quality is low, and the yield is low and the production cost is high, for the following reasons. The polymer beads and the inorganic microparticles projecting from the surface of the optical diffusion layer damage the lens film provided thereon or are easily detached from the optical diffusion layer by an impact or the like. In order to increase the haze for the purpose of improving the optical diffusion function to a sufficient value, the content of the dispersant to be added needs to be high and the optical loss caused by the dispersant is not negligible.
In order to solve these problems, Japanese Laid-Open Patent Publication No. 2002-357706 attempts to express the optical diffusion by embossing and proposes an optical sheet including a substrate layer formed of a transparent synthetic resin and an optical function layer formed on a front surface of the substrate layer. The substrate layer contains a synthetic resin having a glass transition temperature of 100° C. or higher and 180° C. or lower, and tiny concaves and convexes are formed on a rear surface of the substrate layer by embossing. The above-identified publication describes that since the tiny concaves and convexes on the rear surface are formed by embossing, the shape thereof, the surface roughness and the like can be adjusted relatively easily so as to be effective to prevent the surface of the waveguide plate or the like from being damaged. This proposal recommends that the surface roughness (Rz) of the rear surface is 30 μm or greater and 60 μm or less. Today, however, the level of the diffusion function and the luminance improving function desired for the diffusion films is progressively increasing. Diffusion films optically designed and having a controlled structure are now required. It is not sufficient to define these functions by only Rz. Moreover, Rz of 30 μm or greater is too large to completely eliminate the luminance nonuniformity.
Japanese Laid-Open Patent Publication No. 2002-202508 proposes a diffusion film including a diffusion section on a front surface and also on a rear surface, in which either one of the two diffusion sections is formed by embossing. The above-identified publication reports the following: unlike the conventional diffusion films, it is not necessary to perform printing in order to reduce the amount of light emitted by the light emitting diode; and therefore the illuminance is not reduced in the entirety of the screen of the liquid crystal display and it is made possible to illuminate the screen efficiently. The publication also reports that since the diffusion section is formed by embossing, the optical diffusion sheet can be relatively easily formed and the production cost is not costly.
The above-identified publication also describes that in the case where the diffusing section is formed by embossing, the light beam can be diffused more conspicuously where the surface roughness Rz is 5 μm or greater and 50 μm or less. However, it is difficult to provide, at the same time, an optical diffusion function, an improved luminance, a transmittance and the like which are of a sufficient level by merely limiting Rz.
Japanese Laid-Open Patent Publication No. 2003-329812 proposes providing an optical diffusion layer on one surface of a transparent polymer film and forming concaves and convexes on the other surface. The concaved and convexed surface has an arithmetic average roughness of 0.5 μm or less and an average concave-concave/convex-convex interval (Sm) of 80 μm or less. The above-identified publication reports that the concaves and convexes of such a configuration may be directly formed on the transparent polymer film or may be formed on a backcoat layer, which is formed on a surface of the transparent polymer film opposite to the optical diffusion layer.
With this proposal, the optical diffusion function provided by the embossed surface as defined above is insufficient and a separate optical diffusion layer is necessary. This makes the production method troublesome and involves the problems of optical loss and cost increase.
Japanese Laid-Open Patent Publication No. 2004-4417 proposes an optical diffusion sheet including a transparent resin layer on at least one surface of a transparent substrate. The transparent resin layer has tiny concaves and convexes on a surface thereof. The optical diffusion sheet is characterized in that the haze thereof is 30% or higher, the average surface roughness of the central line (Ra: μm) fulfills 0.1≦Ra≦0.4, and the ratio of Ra and the average inter-local peak gap (Sm: μm), i.e., Ra/Sm is 0.05 or less. However, the concaved and convexed surface having such a small value of Ra does not provide a sufficient diffusion function. Thus, this optical diffusion sheet has substantially the same problem as that of the Japanese Laid-Open Patent Publication No. 2003-329812.